pairs unpairs pairkeys pairvalues pairfirst pairgrep pairmap

Transcription

1 NAME SYNOPSIS List::Util - A selection of general-utility list subroutines use List::Util qw( reduce any all none notall first max maxstr min minstr product sum sum0 pairs unpairs pairkeys pairvalues pairfirst pairgrep pairmap DESCRIPTION shuffle uniq uniqnum uniqstr ); List::Util contains a selection of subroutines that people have expressed would be nice to have in the perl core, but the usage would not really be high enough to warrant the use of a keyword, and the size so small such that being individual extensions would be wasteful. By default List::Util does not export any subroutines. LIST-REDUCTION FUNCTIONS reduce The following set of functions all reduce a list down to a single value. $result = reduce { by calling BLOCK in a scalar context multiple times, setting $a and $b each time. The first call will be with $a and $b set to the first two elements of the list, subsequent calls will be done by setting $a to the result of the previous call and $b to the next element in the list. Returns the result of the last call to the BLOCK. is empty then undef is returned. only contains one element then that element is returned and BLOCK is not executed. The following examples all demonstrate how reduce could be used to implement the other list-reduction functions in this module. (They are not in fact implemented like this, but instead in a more efficient manner in individual C functions). $foo = reduce { defined($a)? $a : $code->(local $_ = $b)? $b : undef # first $foo = reduce { $a > $b? $a : $b # max $foo = reduce { $a gt $b? $a : $b 'A'..'Z' # maxstr $foo = reduce { $a < $b? $a : $b # min $foo = reduce { $a lt $b? $a : $b 'aa'..'zz' # minstr $foo = reduce { $a + $b # sum $foo = reduce { $a. # concat $foo = reduce { $a $code->(local $_ = $b) # any $foo = reduce { $a && $code->(local $_ = $b) # all $foo = reduce { $a &&!$code->(local $_ = $b) # none $foo = reduce { $a!$code->(local $_ = $b) # notall # Note that these implementations do not fully short-circuit Page 1

2 If your algorithm requires that reduce produce an identity value, then make sure that you always pass that identity value as the first argument to prevent undef being returned $foo = reduce { $a + $b value # sum with 0 identity The above example code blocks also suggest how to use reduce to build a more efficient combined version of one of these basic functions and a map block. For example, to find the total length of the all the strings in a list, we could use $total = sum map { However, this produces a list of temporary integer values as long as the original list of strings, only to reduce it down to a single value again. We can compute the same result more efficiently by using reduce with a code block that accumulates lengths by writing this instead as: $total = reduce { $a + length $b The remaining list-reduction functions are all specialisations of this generic idea. any my $bool = any { Since version Similar to grep in that it evaluates BLOCK setting $_ to each element in turn. any returns true if any element makes the BLOCK return a true value. If BLOCK never returns true was empty then it returns false. Many cases of using grep in a conditional can be written using any instead, as it can short-circuit after the first true result. if( any { length > ) { # at least one string has more than 10 characters all my $bool = all { Since version Similar to any, except that it requires all elements of to make the BLOCK return true. If any element returns false, then it returns false. If the BLOCK never returns false or was empty then it returns true. none notall my $bool = none { my $bool = notall { Since version Similar to any and all, but with the return sense inverted. none returns true only if no value in causes the BLOCK to return true, and notall returns true only if not all of the values do. Page 2

3 first my $val = first { Similar to grep in that it evaluates BLOCK setting $_ to each element in turn. first returns the first element where the result from BLOCK is a true value. If BLOCK never returns true was empty then undef is returned. $foo = first { $foo = first { $_ > # first defined value # first value which # is greater than $value max my $num = Returns the entry in the list with the highest numerical value. If the list is empty then undef is returned. $foo = max # 10 $foo = max 3,9,12 # 12 $foo maxstr my $str = Similar to max, but treats all the entries in the list as strings and returns the highest string as defined by the gt operator. If the list is empty then undef is returned. $foo = maxstr 'A'..'Z' $foo = maxstr "hello","world" $foo # 'Z' # "world" min my $num = Similar to max but returns the entry in the list with the lowest numerical value. If the list is empty then undef is returned. $foo = min # 1 $foo = min 3,9,12 # 3 $foo minstr my $str = Similar to min, but treats all the entries in the list as strings and returns the lowest string as defined by the lt operator. If the list is empty then undef is returned. $foo = minstr 'A'..'Z' $foo = minstr "hello","world" $foo # 'A' # "hello" Page 3

4 product my $num = Since version Returns the numerical product of all the elements is empty then 1 is returned. $foo = product # $foo = product 3,9,12 # 324 sum my $num_or_undef = Returns the numerical sum of all the elements For backwards compatibility, is empty then undef is returned. $foo = sum # 55 $foo = sum 3,9,12 # 24 $foo sum0 my $num = Since version Similar to sum, except this returns 0 when given an empty list, rather than undef. KEY/VALUE PAIR LIST FUNCTIONS The following set of functions, all inspired by List::Pairwise, consume an even-sized list of pairs. The pairs may be key/value associations from a hash, or just a list of values. The functions will all preserve the original ordering of the pairs, and will not be confused by multiple pairs having the same "key" value - nor even do they require that the first of each pair be a plain string. NOTE: At the time of writing, the following pair* functions that take a block do not modify the value of $_ within the block, and instead operate using the $a and $b globals instead. This has turned out to be a poor design, as it precludes the ability to provide a pairsort function. Better would be to pass pair-like objects as 2-element array references in $_, in a style similar to the return value of the pairs function. At some future version this behaviour may be added. Until then, users are alerted NOT to rely on the value of $_ remaining unmodified between the outside and the inside of the control block. In particular, the following example is UNSAFE: =... foreach (qw( some keys here )) { = pairgrep { $a eq Instead, write this using a lexical variable: foreach my $key (qw( some keys here )) { = pairgrep { $a eq Page 4

5 pairs = A convenient shortcut to operating on even-sized lists of pairs, this function returns a list of ARRAY references, each containing two items from the given list. It is a more efficient version = pairmap { [ $a, $b It is most convenient to use in a foreach loop, for example: foreach my $pair ( ) { my ( $key, $value ) Since version 1.39 these ARRAY references are blessed objects, recognising the two methods key and value. The following code is equivalent: foreach my $pair ( ) { my $key = $pair->key; my $value = $pair->value;... unpairs = Since version The inverse function to pairs; this function takes a list of ARRAY references containing two elements each, and returns a flattened list of the two values from each of the pairs, in order. This is notionally equivalent to = except that it is implemented more efficiently internally. Specifically, for any input item it will extract exactly two values for the output list; using undef if the input array references are short. Between pairs and unpairs, a higher-order list function can be used to operate on the pairs as single scalars; such as the following near-equivalents of the other pair* higher-order = unpairs grep { FUNC # Like pairgrep, but takes $_ instead of $a and = unpairs map { FUNC # Like pairmap, but takes $_ instead of $a and $b Note however that these versions will not behave as nicely in scalar context. Finally, this technique can be used to implement a sort on a keyvalue pair list; = unpairs sort { $a->key cmp $b->key Page 5

6 pairkeys = A convenient shortcut to operating on even-sized lists of pairs, this function returns a list of the the first values of each of the pairs in the given list. It is a more efficient version = pairmap { pairvalues = A convenient shortcut to operating on even-sized lists of pairs, this function returns a list of the the second values of each of the pairs in the given list. It is a more efficient version = pairmap { pairgrep = pairgrep { my $count = pairgrep { Similar to perl's grep keyword, but interprets the given list as an even-sized list of pairs. It invokes the BLOCK multiple times, in scalar context, with $a and $b set to successive pairs of values from Returns an even-sized list of those pairs for which the BLOCK returned true in list context, or the count of the number of pairs in scalar context. (Note, therefore, in scalar context that it returns a number half the size of the count of items it would have returned in list = pairgrep { $a =~ As with grep aliasing $_ to list elements, pairgrep aliases $a and $b to elements of the given list. Any modifications of it by the code block will be visible to the caller. pairfirst my ( $key, $val ) = pairfirst { my $found = pairfirst { Since version Similar to the first function, but interprets the given list as an even-sized list of pairs. It invokes the BLOCK multiple times, in scalar context, with $a and $b set to successive pairs of values from Returns the first pair of values from the list for which the BLOCK returned true in list context, or an empty list of no such pair was found. In scalar context it returns a simple boolean value, rather than either the key or the value found. Page 6

7 ( $key, $value ) = pairfirst { $a =~ As with grep aliasing $_ to list elements, pairfirst aliases $a and $b to elements of the given list. Any modifications of it by the code block will be visible to the caller. pairmap = pairmap { my $count = pairmap { Similar to perl's map keyword, but interprets the given list as an even-sized list of pairs. It invokes the BLOCK multiple times, in list context, with $a and $b set to successive pairs of values from Returns the concatenation of all the values returned by the BLOCK in list context, or the count of the number of items that would have been returned in scalar = pairmap { "The key $a has value As with map aliasing $_ to list elements, pairmap aliases $a and $b to elements of the given list. Any modifications of it by the code block will be visible to the caller. See KNOWN BUGS for a known-bug with pairmap, and a workaround. OTHER FUNCTIONS shuffle = Returns the values of the input in a random = shuffle # in a random order uniq = Since version Filters a list of values to remove subsequent duplicates, as judged by a DWIM-ish string equality or undef test. Preserves the order of unique elements, and retains the first value of any duplicate set. my $count = In scalar context, returns the number of elements that would have been returned as a list. The undef value is treated by this function as distinct from the empty string, and no warning will be produced. It is left as-is in the returned list. Subsequent undef values are still considered identical to the first, and will be removed. uniqnum = Since version Page 7

8 Filters a list of values to remove subsequent duplicates, as judged by a numerical equality test. Preserves the order of unique elements, and retains the first value of any duplicate set. my $count = In scalar context, returns the number of elements that would have been returned as a list. Note that undef is treated much as other numerical operations treat it; it compares equal to zero but additionally produces a warning if such warnings are enabled (use warnings 'uninitialized'; ). In addition, an undef in the returned list is coerced into a numerical zero, so that the entire list of values returned by uniqnum are well-behaved as numbers. Note also that multiple IEEE NaN values are treated as duplicates of each other, regardless of any differences in their payloads, and despite the fact that 0+'NaN' == 0+'NaN' yields false. uniqstr = Since version Filters a list of values to remove subsequent duplicates, as judged by a string equality test. Preserves the order of unique elements, and retains the first value of any duplicate set. my $count = KNOWN BUGS RT #95409 In scalar context, returns the number of elements that would have been returned as a list. Note that undef is treated much as other string operations treat it; it compares equal to the empty string but additionally produces a warning if such warnings are enabled (use warnings 'uninitialized';). In addition, an undef in the returned list is coerced into an empty string, so that the entire list of values returned by uniqstr are well-behaved as strings. If the block of code given to pairmap contains lexical variables that are captured by a returned closure, and the closure is executed after the block has been re-used for the next iteration, these lexicals will not see the correct values. For example: = pairmap { my $var = "$a is $b"; sub { print "$var\n" ; one => 1, two => 2, three => 3; $_->() Will incorrectly print three is 3 three is 3 three is 3 This is due to the performance optimisation of using MULTICALL for the code block, which means that fresh SVs do not get allocated for each call to the block. Instead, the same SV is re-assigned for each iteration, and all the closures will share the value seen on the final iteration. Page 8

9 To work around this bug, surround the code with a second set of braces. This creates an inner block that defeats the MULTICALL logic, and does get fresh SVs allocated each time: = pairmap { { my $var = "$a is $b"; sub { print "$var\n"; one => 1, two => 2, three => 3; This bug only affects closures that are generated by the block but used afterwards. Lexical variables that are only used during the lifetime of the block's execution will take their individual values for each invocation, as normal. uniqnum() on oversized bignums Due to the way that uniqnum() compares numbers, it cannot distinguish differences between bignums (especially bigints) that are too large to fit in the native platform types. For example, my $x = Math::BigInt->new( "1" x 100 ); my $y = $x + 1; say for uniqnum( $x, $y ); Will print just the value of $x, believing that $y is a numerically- equivalent value. This bug does not affect uniqstr(), which will correctly observe that the two values stringify to different strings. SUGGESTED ADDITIONS The following are additions that have been requested, but I have been reluctant to add due to them being very simple to implement in perl # How many elements are true sub true { scalar grep { # How many elements are false SEE ALSO COPYRIGHT sub false { scalar grep Scalar::Util, List::MoreUtils Copyright (c) Graham Barr <gbarr@pobox.com>. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. Recent additions and current maintenance by Paul Evans, <leonerd@leonerd.org.uk>. Page 9